Fabric filters are frequently used in gas cleaning systems. Such fabric filters typically include a number of filter bags where gas may enter while particles or dust entrained in the gas are stopped by the filter fabric and end up on the outside of the filter bags. When such a fabric filter has been in use for a while, the outside surfaces of the filter bags are more or less covered with particles or dust and the filter bags then need to be cleaned in order to maintain a proper passage of gas through the fabric. The cleaning of the filter bags may take place by providing a pulse of compressed air stored in a tank to the inside of the filter bags to quickly stretch the fabric thereby “shaking off” the particles attached to the outside surfaces of the filter bags. In order to be able to provide the pulses of compressed air to the filter bags, a valve arrangement may be provided. The pulses have the function first to shake the bags. Secondly the pulse is to provide a gas flow in the opposite direction of the flue gas flow. The reversed flow serves to blow particles through the filter media and also transport the particles away from the filter material in the opposite direction of the flue gas flow. The third function of the pulse is to provide soft landing. Soft landing is that the filter material should return to the cage in a controlled manner and not hit the cage too hard. Soft landing is a means to both extend the bag life time and decrease emissions. The first objective is achieved by sizing the pressure tank large enough (APEX, investment cost) to maintain proper tank pressure during fulfillment of the first and second objective. To meet the second and third objective the valve opening time is prolonged. The prolonged opening time increases the operational cost.
Current sizing of the air volume needed for cleaning is based on the area of fabric for the number and length of bags that should be cleaned by one valve. By emptying the tank, soft landing is achieved. This will consume the whole tank volume (end pressure after emptying the tank is approximately 0.5 bar overpressure). To get higher bag pulse cleaning pressure tank volume can be increased. Increasing tank volume increases cleaning performance but introduces significant cost increase in both investment and operational cost. Increasing tank volume gives also longer valve opening time to achieve a soft landing, which may be more soft than needed. But with current deign and control of flow, the air consumption will increase, causing higher cost.
A method for cleaning the filter bags of a bag-filter installation is disclosed in EP 0670749 A1, where the filter bags are arranged in a plurality of rows and the dust content of the filtered gas is monitored by means of a monitoring unit, cleaning is carried out by blowing a pressure medium into the filter bags. The pressure medium is blown into the filter bags row by row, the filter bags in one and the same row being cleaned simultaneously, and the rows being successively cleaned at a time interval exceeding the time it takes for conveying the filtered gas from the cleaned row to the monitoring unit.
A method and apparatus for cleaning the cloth in fabric filters is disclosed in WO 1993013845 A1, where an apparatus for cleaning filter bags by reverse flushing of the filter bags with a gas is proposed, which filter bags are arranged in sections comprising one or more filter bags, which sections are connected to the cleaning apparatus by ducts which are arranged in a circle around the apparatus. The apparatus comprises a circular, stationary port tube having ports disposed at each of the ducts, a rotating nozzle tube having a nozzle, which nozzle tube is disposed on the inside of the port tube, a venturi/ejector provided on the outside of the port tube and connected with the nozzle tube by means of an arm, which nozzle tube and ventur/ejector are adapted for stepwise rotation with the aid of an advancing means. There is also described a method which utilizes the apparatus.
A device for cleaning filter elements is disclosed in EP 0891215 B1, where the device for cleaning filter elements, such as filter bags, having upper open ends, in a filter installation for filtering of polluted gas. The gas passing, during operation, from the outside and through said filter elements, in which installation filter elements are juxtaposed in a plurality of essentially parallel rows. The device comprises an essentially horizontal distribution pipe, which extends essentially in parallel with the rows of filters elements and is adapted to be connected to a pressure medium source via a valve. The device further comprises a plurality of nozzle elements, which are connected to the distribution pipe. The nozzle elements are adapted to direct pressure medium pulses (P) into the upper open ends of the filter elements in at least two juxtaposed rows of filter elements.
A filtration apparatus for removing entrained dust particles from a gas stream is disclosed in U.S. Pat. No. 4,283,208 A, wherein the apparatus comprises a housing having an inlet port for dust entrained gas and an outlet port for the exhaust of clean gas therefrom, an apertured tube sheet intermediate the inlet and outlet ports arranged to divide the housing into inlet and outlet sections, a plurality of porous filter bags each having an open end and a closed end with the open end thereof attached to the tube sheet around each aperture thereof to permit the flow of clean gas therethrough while retaining dust particles on the surface of the filter, cleaning means for said filter bags comprising a linear force electromagnetic motor having oppositely extending slider rods connected to said filter bags so as to move horizontally to shake each filter bag, a source of electric current connected to the linear force motor, and means for controlling the current being supplied to the linear force motor to modulate the frequency and amplitude of movement of the slider rods to vary the cleaning effect of said cleaning means.